This disclosure generally relates to graphic image displays, and especially relates to graphic image displays lit by both backlight and ambient light.
Graphic image displays that rely on backlighting and/or ambient lighting are used in a variety of applications, such as outdoor commercial posters or signs, warning signs, labels for appliances, or electronic devices instrument clusters, and automotive switches and buttons, etc. The desired characteristics of the graphic image display for optimum performance include good hiding power for the backlight source, high light transmittance, good image contrast, low glare of ambient light, wide or controlled viewing angles, and the durability of the graphic image if it is printed.
In order to provide a uniform intensity profile across the surface of backlit displays, optical sheet(s) with certain levels of light diffusion are used to “spread-out” or diffuse the incident light from the localized backlight sources so that the “bright spots” due to the backlight source are not seen. Such light diffusion capability of the optical sheet, also referred to as hiding power, can be characterized by the haze of the optical sheet. In traditional art, adding diffusing particles to the optical sheet improves hiding power. However, the transmittance of the sheet is reduced due to the addition of diffusing particles.
Another characteristic is the image contrast of the display, i.e., the brightness contrast between the bright and illuminated areas and the surrounding darker areas within the same image. Ambient light is incident on a display from the viewer's side and reflected at the exterior surface of the display. The reflected ambient light is superimposed on the displayed information lit by the backlight source, resulting in a reduced contrast for the viewer. Typically, a light-absorbing ink is used in the image carrying layer to absorb the majority of the ambient light (and the backlight) incident upon the areas surrounding the illuminated window areas of the backlit graphics to improve the image contrast. Moreover, in traditional art, an ink layer that partially reflects and partially transmits the light can be applied only in front of the illuminated window areas of the backlit graphics to partially reflect the ambient light incident upon the window areas in order to further improve the lighting contrast between the window areas and surrounding darker areas in presence of the strong ambient light (e.g., direct sun light). However, the transmittance of the printed graphic image to the incident backlight in the window areas is reduced due to presence of the partially reflecting ink on the surface, which may require increasing the power of the backlight sources to compensate for reduced backlighting brightness when the ambient light is not strong (e.g., during cloudy days), which renders additional cost due to the ink layer itself and the increased power consumption of the backlight sources.
Yet another characteristic of the graphic image display is the viewing angles of the display. For outdoor commercial posters or signs, or warning sign applications, wide viewing angles are generally desired. Adding light diffusing particles into the sheet bearing the graphic image is a traditional way of achieving the wide viewing angles. Again, however, added diffusing particles reduce the transmittance of the backlight and increase the cost.
Thus, there is a need in the industry for the graphic displays with enhanced image contrast and hiding power, i.e., without compromising the transmittance of the backlight for the graphic display. There is also a need to reduce the cost of the display by removing or reducing the amount of the diffusing particles in the optical sheet, and/or removing the above mentioned partially light-reflecting ink layer without sacrificing the overall performance of the graphic display.